[libc++] Fix a typo in reverse_iterator::operator=.

We should be checking `is_assignable<It&, ...>`.
`is_assignable<It, ...>` checks for an rvalue left-hand side, which
is basically never assignable-to.
Found while looking into https://cplusplus.github.io/LWG/issue3435 .

Differential Revision: https://reviews.llvm.org/D117660